Chromatic Aberration

More than 200 years ago, Newton showed that white light was composed of multiple wavelengths. Simple lenses will refract (bend) light differentially as a function of wavelength. Short (blue appearing) wavelengths are refracted more than long (red appearing) wavelengths. Consequently, lenses like the one shown above will not image light all in one place.

You may be interested to know that the human eye lens exhibits chromatic aberration. Fortunately, a yellow pigment in the fovea called the macula lutea helps to protect us from this problem. Yellow pigments absorb blue light.